Hu protein is expressed in all embryonic and adult neurons, and is one of the earliest markers of neuronal differentiation. Hu is a member of a family of RNA-binding proteins that are homologous with the product of the elav gene in Drosophila. It has been known for more than a decade that mutations in elav result in specific loss of neurons, implying that ELAV is necessary for neuronal development. Recently, we have shown that misexpression of Hu in embryonic neural crest cells can promote the expression of neuronal traits. However, the mechanism by which Hu/ELAV protein elicits neuronal differentiation has remained elusive. Recently, ELAV was shown to be necessary for the neuron- specific splicing of neuroglian mRNA, suggesting that ELAV, and presumably the Hu proteins, mediate their effects by regulating the neuron-specific splicing of gene products required for neuronal differentiation. It is known that heterogeneous nuclear ribonucleoproteins (hnRNPs), abundant and ubiquitously expressed RNA- binding proteins, are involved in RNA splicing, but it is not clear how cell-type specific splicing is mediated. We have recently been able to show, however, that Hu proteins associate with hnRNPs in an RNA- dependent manner. Our preliminary results suggested a new hypothesis that binding of Hu proteins to hnRNPs confer neuron-specificity to the function of hnRNP complexes. We propose to confirm and extend our preliminary results, and thereby provide the foundation for future research proposals to establish the role of Hu proteins in the post- transcriptional regulation of neuronal differentiation.